Existing published pediatric epidemiologic CFS studies are similar to the first generation of adult CFS prevalence studies in that they have had either poor sampling plans (e.g., recruitment at medical centers; Nijhof et al., 2011), or systematic biases that excluded certain people such as youth of lower socioeconomic status and those of color who were less likely to have access to health care (Dobbins et al., 1997), or failed to include a medical examination (Jones et al., 2004). We will determine the prevalence of pediatric CFS in a demographically diverse sample of participants unbiased by illness, help-seeking behaviors, or differential access to the health care system. In addition, we will assess orthostatic intolerance (OI) symptoms in a community-based sample of children with pediatric CFS, who are unbiased by help-seeking behaviors, or differential access to the health care system. In the proposed study, the quantifiable response to a mental task undertaken during orthostatic stress (a) will operationally define central fatigue and neurocognitive impairment and (b) will be applied as a biomarker for community-based CFS when compared to a community-based control group. We believe that cognitive fatigue, cognitive loss, and dizziness are a function of abnormalities in cerebral blood flow as it relates to total cardiac output, total blood volume, regional blood flow, and blood volume distribution during the orthostatic stressor. In addition, we will be able to resolve discrepant findings regarding cerebral blood flow, which, we believe, are due to the differing ways of measuring the response of cerebral blood flow velocity to arterial pressure. In summary, this proposed study will determine the prevalence of pediatric CFS in a community-based sample, as well as the relative frequency of CFS among various groups (e.g., different age groups, genders). This study will also identify the prevalence of orthostatic abnormalities among youth with CFS and controls and will examine its relationship with neurocognitive functioning.